drm/bridge: sii902x: add support for DRM_BRIDGE_ATTACH_NO_CONNECTOR

This adds support for DRM_BRIDGE_ATTACH_NO_CONNECTOR by adding the
bridge get_edid() and detect() callbacks after refactoring the connector
get_modes() and connector_detect() callbacks.

In order to keep the bridge working, extra code in get_modes() has been
moved to more logical places.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220113144305.1074389-1-narmstrong@baylibre.com
Reviewed-by: Robert Foss <robert.foss@linaro.org>